The molecular structures and conformational compositions of 1,3 cyclohexanedione and 1,4 cyclohexanedione as determined by gas-phase electron diffraction and theoretical calculation

The molecular structures of 1,3 and 1,4 cyclohexanedione have been investigated in the gas phase at nozzle temperatures of 379 K and 383 K respectively. These two molecules exist in conformation mixtures of: twist (70(9)%) and chair (30(9)%) forms for 1,4 cyclohexanedione; and chair (60(13)%) and boat (40(13)%) forms for 1,3 cyclohexanedione. The results are consistent with theoretical MP2 and B3LYP calculations using 6-311G(d,p) basis set where the stable forms found were: twist and chair for 1,4 cyclohexanedione and chair, boat and twist for 1,3 cyclohexanedione. The geometrical parameter values (rg   and ∠∠α) and their error limits (3σ) obtained from least squares refinements for the twist form of 1,4 cyclohexanedione are : r(CH)av = 1.116(5) Å, r(CO) = 1.220(2) Å, r(C1C2) = 1.528(8) Å, r(C2C3) = 1.535(17) Å, ∠∠C1C2C3 = 113.3(5) Å, ∠∠C6C1C2 = 117.9(10)°, τC6C1C2C3 = 24.6(6)° and Flap (angle between planes formed by atoms C3, C4, C5 and C3, C5 and the mid-point of atoms C6 and C2) = 180°. and for the chair form of 1,3 cyclohexanedione: r(CH)av = 1.105(5) Å, r(CO) = 1.220(2) Å, r(C1C2) 1.528(2) Å, r(C3C4) = r(C1C2) − 0.008 Å, r(C4C5) = r(C1C2) + 0.010 Å, ∠∠C1C2C3 = 114.0(8)°, ∠∠C6C1C2 = 116.2(5)°, τC6C1C2C3 = 40.1(2)°, and Flap(angle between planes C4C5C6 and C4C6C2) = 142.8°(assumed).

► The molecular structures of 1,3 and 1,4 cyclohexanedione have been studied in the gas phase.
► For 1,4 cyclohexanedione a mixtures of twist (70(9)%) and chair (30(9)%) forms was observed.
► For 1,3 cyclohexanedione a mixture of chair (60(13)%) and boat (40(13)%) forms was observed.
► The results are consistent with theoretical MP2 and B3LYP calculations using 6-311G(d,p) basis set.